Yes, it's always possible to find Mac ROMs on the internet, but these are not necessarily legal downloads. The advantage of the Mac OS ROM update is that it comes straight from Apple and can be downloaded legally.

So what I'm wondering here is if a patch to the Mac OS ROM update download is legal to provide to the public - as it would get around needing an existing old Mac to extract the ROM file with TomeViewer. Or would the nature of this patch still be illegal? Its sole purpose is to enable people to obtain a ROM file without needing to own a Mac.

An alternate approach would be to hack the Tome format and provide a utility that extracts any Tome - this would have a multitude of uses and no longer have a purpose specific to this file. But this is a lot more difficult than making a patch.